Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the reduction of NOx.
The total particulate matter emissions of diesel exhaust are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The soluble organic fraction is sometimes referred to as the volatile organic fraction (“VOF”), which terminology will be used herein. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulate matter is the so-called sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel. Small proportions of SO3 are formed during combustion of the diesel, which in turn combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of the total particulate matter.
One after treatment technology in use for high particulate matter reduction is the diesel particulate filter. There are many known filter structures that are effective in removing particulate matter from diesel exhaust, such as honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust. The filter is a physical structure for removing particles from exhaust, and the accumulating particles will increase the back pressure from the filter on the engine. Thus the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure.
Filters coated with Selective Catalytic Reduction (SCR) catalysts may be considered for the reduction of size and cost of the next generation diesel emissions control systems for CO, HC, NOx and particulate matter. In SCR processes, NOx is reduced with ammonia (NH3) to nitrogen (N2) over a catalyst typically composed of base metals. The application of SCR catalysts to high porosity filter substrates has allowed a reduction in system size while maintaining filtration efficiency and NOx conversion. High porosity filters with large mean pore size (20 μm or more) and narrow pore size distribution have shown to be advantageous because they allow the best catalyst utilization with the lowest back pressure increase.
Catalyzed wall flow filters containing a catalyst that promotes SCR of NOx assume two functions: removal of the particulate component of the exhaust stream and conversion of the NOx component of the exhaust stream to N2. SCR-coated wall flow filters that can achieve NOx reduction goals require a sufficient loading of SCR catalyst composition on the wall flow filter under the usual space constraints in a vehicle. The gradual loss of the catalytic effectiveness of the compositions that occurs over lifetime through exposure to certain deleterious components of the exhaust stream or high temperatures augments the need for higher catalyst loadings of the SCR catalyst composition. However, preparation of coated wall flow filters with higher catalyst loadings can lead to unacceptably high back pressure within the exhaust system. An increase in backpressure can have an adverse impact on fuel efficiency.
An additional aspect for consideration in coating the wall flow filter is the selection of the appropriate SCR catalyst composition. First, the catalyst composition must be thermally durable so that it maintains its SCR catalytic activity even after prolonged exposure to higher temperatures that are characteristic of filter regeneration. For example, combustion of the soot fraction of the particulate matter often leads to temperatures above 700° C. and higher. Such temperatures render many commonly used SCR catalyst compositions such as mixed oxides of vanadium and titanium less catalytically effective. Second, the SCR catalyst compositions preferably have a wide enough operating temperature range so that they can accommodate the variable temperature ranges over which the vehicle operates. Temperatures below 300° C. are typically encountered, for example, at conditions of low load, or at startup. The SCR catalyst compositions are preferably capable of catalyzing the reduction of the NOx component of the exhaust to achieve NOx reduction goals, even at lower exhaust temperatures, particularly when the SCR catalyst is disposed on a filter substrate such as a wall flow filter. In general the SCR catalyst should have a high specific activity combined with a high hydrothermally stability.
For Euro 6 emission regulations, the particulate emissions will be measured on a particle number basis rather than a particle mass basis. The move to particle number count for particulate matter emissions is seen as a tighter restriction on emissions. Large mean pore size, high porosity filter are not favored in number based filtration efficiency measurements and the move has been to lower mean pore size filters in order to meet the new regulations. However, as stated before, lower mean pore size filter materials are not favored for Selective Catalytic Reduction Filters (SCRF) applications. Thus there is a need to raise the filtration efficiency of large mean pore sized, high porosity filters, while retaining the high pore volume and pore accessibility needed for high SCR catalyst loading.